Ultrathin high-performance hollow fiber membranes

ABSTRACT

A process for forming ultrathin dense-layer asymmetric hollow fiber membranes with a dense layer of less than 500 Å from a binary solution system comprising a polymer and a solvent. In this process, the spinning polymeric solution has a high viscosity and exhibits chain entanglement at the spinning temperature. The solubility parameter difference between the bore fluid and the spinning dope is less than 2.5 (cal/cm 3 ) 0 .5 and the volume ratio of bore-fluid flow rate to the dope flow rate is between 0.45 to 0.75. The dope is wet-spun into hollow fibers using water as external coagulant. The ultrathin dense-layer asymmetric hollow fiber membranes are suitable for air and other gas separations.

References from the scientific and patent literature cited herein arehereby incorporated in their entirety by reference.

BACKGROUND OF THE INVENTION

This invention relates to the field of asymmetric hollow fibers,particularly those that comprise an ultrathin selective layer for airand gas separations.

In creating a gas or liquid separation membrane, it is desirable to haveboth a high rate of permeation or throughput and a high separationfactor or selectivity. This combination of characteristics permits theeffective separation of a relatively large volume of fluid per unittime. The phase inversion process is one of the most important means toprepare asymmetric membranes for air separation. The resultant membraneshave a dense skin layer that is integrally bonded in series with a thickporous substructure. The skin and the substructure are composed of thesame material.

The skin layer, which contains the effective separating layer, is one ofthe key elements in determining the membrane permeability andselectivity. To have a high-performance air-separation membrane, thisskin layer has to be as thin as possible and must contain a minimum ofdefects.

U.S. Pat. No. 4,871,494 issued to Kesting, et al. describes a processfor forming asymmetric gas separation membranes having graded densityskins. This process comprises dissolving a glassy polymer in a Lewisacid: base complex solvent system wherein the Hildebrand parameters ofthe solvent species and the polymer are within 1.5. The spinning dopeprepared by this method comprises a polymer and a solvent mixture, butthe solvent mixture further contains two components.

U.S. Pat. No. 4,902,422 issued to Pinnau and Koros describes solutionformulations to prepare defect-free ultrahigh flux flat asymmetricmembranes using a forced convective drying process. In order to form theskin layer, this process requires solvents which either have low boilingpoints or high vapor pressures in the solution system. The cast solutionprepared by this method has multiple components, and at least consistsof a polymer, a low boiling point solvent and a high boiling pointsolvent.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an asymmetric hollowfiber membrane having both good selectivity and permeability for air andgas separations.

It is another object of the present invention to provide a method forimproving the permeability of asymmetric hollow fiber membranes.

It is a further object of the present invention to provide a method formaking highly permeable asymmetric hollow fiber membranes usingbiodegradable or environmental friendly solvents, such as NMP(N-methyl-2-pyrrolidone).

Other objects and advantages of the present invention will be apparentto those skilled in the art from the following description and theappended claims.

SUMMARY OF THE INVENTION

The invention is a semi-permeable asymmetric hollow fiber membranecomprising a glassy polymeric material with a porous inner layer and adense external layer of less than 500 Å. After applying a siliconecoating, the ultrathin dense layer can selectively separate one gaseouscomponent from other gaseous components in a gas mixture. Both of theinner layer and dense external layer are comprised of the same material.

In another aspect, the invention is a process for preparing asemi-permeable hollow fiber membrane comprising:

A) dissolving a polymer possessing an equilibrium water content at about25° C. of less than 2 w/w % in a solvent and forming, after continuousstirring, a homogeneous fluid, to be known as the spinning dope,possessing sufficient viscosity and exhibiting chain entanglement at thetemperature at which the dope is to be spun;

B) extruding the said polymer-solvent solution through a spinneret (asshown in FIG. 1) into a hollow fiber membrane using water as an externalcoagulant and using a bore fluid having the following properties.

1. the solubility parameter difference between the bore fluid and thespinning dope is less than 2.5 (cal/cm³)⁰.5,

2. the volume ratio of bore-fluid flow rate to the dope flow rate isbetween 0.45 to 0.75;

C) immersing the spinneret in the water, both inner and externalcoagulations occurring simultaneously;

D) desolvating the asymmetric gas separation membrane thus formedthrough a water bath;

E) washing the membrane at ambient temperature in a polar medium; and

F) drying the membrane at temperatures of from about ambient to about20° C. below the glass transition temperature.

The asymmetric hollow fiber membrane of this invention demonstratessurprisingly high gas separation factors and high gas permeabilities forthe separation of at least one gaseous component from other gaseouscomponents in a gas mixture. In particular, the membranes of thisinvention are useful for the separation of oxygen and nitrogen from airas well as for the separation of hydrogen from gas mixtures containinghydrogen. The asymmetric hollow fiber membrane of this invention has aselectivity above 90% of the inherent selectivity of the glass material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram of the hollow fiber spinning apparatus

FIG. 2 is the solution viscosity of polyethersulfone (PES) inN-methyl-2-pytrolidone (NMP) measured at 25° C. using a Brookfield®cone-and-plate viscometer (Model: FIB DV-III). This viscosity dataexhibits a visible chain entanglement phenomenon, shown by the rapidchange in slope, at a concentration of about 35 wt %.

FIGS. 3A-3B show electron micrographs at magnifications of 50 (3A) and750 (3B) of a cross-section of an ultrathin asymmetric hollow fibermembrane wet-spun from a 35% PES/NMP dope.

FIG. 4 is an electron micrograph at a magnification of 80,000 of across-section near the outer skin of an ultrathin asymmetric hollowfiber membrane wet-spun from a 35% PES/NMP dope.

FIG. 5 is a scanning electron micrograph (SEM) at a magnification of50,000 of the outer skin of an ultrathin asymmetric hollowfiber-membrane wet-spun from a 35% PES/NMP dope.

FIGS. 6A-6B show electron micrographs at a magnification of 50,000 ofthe outer skin of asymmetric hollow fiber membranes dry-jet wet-spun(6A) and wet-spun (6B) from a 30% PES/NMP dope.

The accompanying drawings which are incorporated into and constitute apart of the description of the invention, illustrate the embodiments ofthe invention and serve to explain the principles of the invention. Itis to be understood, however, that the drawings are designed forpurposes of illustration only, and not as a definition of the limits ofthe invention for which reference should be made to the claims appearingat the end of the description.

DETAILED DESCRIPTION OF THE INVENTION

The invention resides in part in a process for forming ultrathindense-layer asymmetric hollow fiber membranes. The process comprises inits most important aspect dissolving a polymer in a solvent to form ahomogenous dope possessing sufficient viscosity and exhibiting chainentanglement at the spinning temperature. The dope is then extruded toform a hollow fiber membrane using water as an external coagulant and abore fluid comprising with and a water miscible solvent. The solubilityparameter difference between the bore fluid and the spinning dope isless than 2.5 (cal/cm³)⁰.5 and the volume ratio of bore-fluid flow rateto the dope flow rate ranges from 0.45 to 0.75.

After extrusion, the fiber is desolvated by allowing it to stand in apolar solvent, preferably water or methanol. A preferred desolvatingprocess it to allow the fiber membrane to stand in the coagulating bathfor some time, then washing it with methanol.

After the hollow fiber membrane is formed, it is preferably coated toseal surface defects. Preferably the fiber membrane is sealed by coatingit with silicone by methods known in the art.

Preferred polymers for use in the process are selected from the groupconsisting of polyimide, a fluoropolymer, polysulfone, polyethersulfone,polyarylate, polycarbonate, a polybenzimidazole, polyetherketone,polyetherether ketone, and a polyester. Of fluoropolymers, the preferredone is a fluoropolyimide. Most preferably, the polymer ispolyethersulfone.

The solvent for the dope is preferably a N-(C₁ -C₃ alkyl)-2-pyrolidone,most preferably N-methyl-2-pyrolidone.

The ratio of solvent to water in the bore fluid can range from 80/20 to95/5, most preferably 80/20 to 90/10. The preferred solvent for the borefluid is a N-(C₁ -C₃ alkyl)-2-pyrolidone, most preferablyN-methyl-2-pyrolidone.

A preferred embodiment of the invention is one wherein the dope is 35/65polyethersulfone/N-methyl-2-pyrolidone and the bore-fluid is 80/20N-methyl-2-pyrolidone/water.

The invention further resides in hollow fiber membrane produced by theprocess; generally uncoated membranes will exhibit a selectivitycharacterized in that the O₂ /N₂ selectivity is at least 0.96. Coatingthe membranes with silicone greatly increases the selectivity, generallyby 5-6 fold. The membranes of the invention have excellent permeance;the permeance observed for O₂ is about 10.1×10⁻⁶ cc(at StandardTemperature and Pressure)/cm² -sec-cm Hg.

EXAMPLE 1

FIG. 1 is a schematic diagram of the hollow fiber spinning apparatus.FIG. 2 shows the viscosity of a PES/NES spinning dope as a function ofPES solid concentration. This dope exhibits chain entanglement at 35/65ratio of PES/NMP. A formulated 35/65 PES/NMP dope was therefore used inthis example. This dope was fed under nitrogen pressure and an 80/20NMP/water bore fluid was fed by a 500D Syringe Pump, made by ISCO. Theaccuracy of this ISCO precision pump was ±0.5% of the flow rate. Thespinning dope and bore fluid met at the tip of the spinneret which wasimmersed in a water coagulation bath at 25° C. No nascent fibers wereextended by drawing; the take-up velocity of the hollow fiber was nearlythe same as the free falling velocity in the coagulation bath. Afterformation of the hollow fiber, the fibers were stored in the water bathfor at least one day and then transferred to a tank containing freshmethanol for at least one hour to completely remove any residual NMP.Hollow fibers thus treated were used for further study. In the aboveprocess, the solubility parameter difference between the 80/20 NMP/H₂ Oand 35/65 PES/NMP was 2.19 (cal/cm³)⁰.5 and the bore fluid rate was 0.1cc/min. Other information relating to the process conditions andparameters are given in Table 1. Membrane samples for SEM study werefirst immersed in liquid nitrogen and fractured, and then sputtered withgold using a Jeol JFC-1100E Ion Sputtering Device. We employed a Jeol®JSM U3 electron microscope and a Hitachi® S-4100 field emission scanningelectron microscope to investigate fiber morphology.

Two to five fibers with a length of 10 cm were assembled into modules.One end of the bundles was sealed with a 5-minute rapid solidifyingepoxy resin (Araldite®, Switzerland), while the shell side of the otherend was glued onto an aluminum holder using a regular epoxy resin(Eposet®). It took 8 hours to fully cure the Eposet® resin. The preparedmodule was fitted into a stainless steel pressure cell for the gaspermeation measurement at 200 psi (13.6 bar).

The permeances, P/L, of gases through the hollow fiber module weredetermined using a bubble-flow meter and calculated using the followingequation: ##EQU1## where P=permeability of the separation layer;t=effective length of the fibers;

ΔP=transmembrane pressure drop;

A=membrane effective surface area;

Q=the gas flux reading;

n=number of tested fibers;

D=outer diameter of the fibers.

We use GPU as the gas permeation units, and one GPU is equal to 1×10⁻⁶cm³ (STP)/cm2-sec-cm Hg. The ideal separation factor of an asymmetricmembrane can be determined from: ##EQU2## where _(A) and _(B) denotedifferent gases.

The effective skin layer thickness is estimated from the oxygenpermeability and the pressure-normalized flux of oxygen from thefollowing equation: ##EQU3## P_(O).sbsb.2 in equation (3) is thepermeability of O₂ in the material.

(P/L)_(O).sbsb.2 is calculated as in equation (1).

L is the effective skin layer thickness.

The selectivities of polyethersulfone (PES) for O₂ /N₂ can be, forinstance, 6.1 with a permeability of 0.51 Barrer at 30° C., or 5.1 witha permeability of 0.81 Barrer at 50° C. In addition, one may calculatepermeability of O₂ at 25° C., which maybe about 0.44 Barrer by using anArrhenius relationship between permeability and temperature. Oxygenpermeability is further described in Journal of Membrane Science 133(1997) 161-175, which is incorporated by reference herein.

Assembled hollow fiber modules were further immersed in a coatingsolution containing 3 wt % polydimethysiloxane (Sylgard-184) in n-hexanefor 5 minutes in order to seal the membrane defects. The coated fibermembrane module was left to stand for 48 hours to cure the siliconerubber coating at room temperature before conducting permeation tests.

Table 4 summarizes the separation performance of the hollow fiber forair separation at 25° C. The uncoated fiber has an O₂ /N₂ selectivity ofabout 0.96 and this value is significantly improved to 5.80 after asilicone coating. The invented fiber has a permeance of 10.13×10⁻⁶cc(STP)/cm² -sec-cm Hg for O₂ at room temperature and the calculateddense layer thickness is 434 Å using Eq. 3. (STP, standard temperatureand pressure). FIGS. 3A and 3B depict SEM pictures of the fibercross-section and show that the region near the inner surface exhibitsof no finger-like voids and no dense layer. FIG. 4 illustrates thedetailed structure near the outer layer at a magnification of 80,000 andindicates this layer consists of nodules. The outermost skin has athickness of approximately 600-800 Å and the substructure beneath theoutermost skin is quite porous and has a tiny finger-like structure.Since the gold coating thickness on SEM samples is around 100-150 Å,this SEM picture suggests that the selective layer should be less than600 Å. FIG. 5 reveals the outer surface morphology and shows no visibledefects.

EXAMPLE 2

Except the bore fluid concentration, air-gap distance and flow rate, allother process conditions are substantially the same as in Example 1. Thebore fluid in this example is 60/140 NMP/water and the bore fluid rateis 0.05 cc/min. The solubility parameter difference between the 60/40NMP/H₂ O and 65/35 PES/NMP is 4.63 (cal/cm³)⁰.5. Other informationprocess conditions and parameters are given in Table 1.

Table 3 compares gas separation performance of as-spun fibers before andafter the silicone rubber coating and both show poor separationperformance. Before silicone coating, wet-spun and dry-jet wet-spunfibers have O₂ /N₂ selectivities in the range of 0.93 to 0.96. Aftercoating, the wet-spun fibers have a slightly better O₂ /N₂ selectivitythan that of the dry-jet wet-spun fibers. However, both haveselectivities well below 90% of the inherent selectivity of PES.

EXAMPLE 3

A formulated 30/70 PES/NMP dope was used as the spinning dope and a40/60 NMP/water mixture was used as bore fluid. The dope viscosity is8,493 cp which is below the critical viscosity of approximately 34,000cp. Other information about process conditions and parameters are givenin Table 1. The solubility parameter difference between the 40/60 NMP/H₂O and 30/70 PES/NMP is 6.83 (cal/cm³)⁰.5. Table 3 compares gasseparation performance of as-spun fibers before and after the siliconerubber coating and both show poor separation performance. FIG. 5 showsthe SEM pictures of external surface of these fibers at 50,000magnification. Both fibers have visible defects in their external skins.

                  TABLE 1                                                         ______________________________________                                        Process Parameters and Spinning conditions for 30 and 35 wt %                   PES/NMP Dopes.                                                                ID                  Example 3                                                                              Example 2                                                                            Example 1                               ______________________________________                                        Polymer concentration PES/NMP (by                                                               30%      35%      35%                                         Viscosity (poise) at 10 sec.sup.-1 8493 34,444 34,444                         Dope pressure, psi 10 25 25                                                   Dope fluid rate, gram/min 0.32 0.234 0.216                                    Spinning speed, cm/min 81 41.4 67.8                                           Spinning temperature, ° C. 25 25 25                                    Bore fluid composition (NMP/H.sup.2 O 40/60 60/40 80/20                       by weight)                                                                    Bore fluid flow rate (cc/min) 0.083 0.05 0.1                                  External coagulant Water Water Water                                          Coagulation bath temperature ° C. 25 25 25                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Permeance of PES Wet-spun fibers (spun from 35 wt % PES/NMP                     dopes using 80/20 NMP/H.sub.2 O as the bore fluid)                                   O.sub.2 permeance                                                                         N.sub.2 permeance                                                                         Selectivity O.sub.2 /N.sub.2                 ______________________________________                                        Before Silicone                                                                        258.42      269.19      0.96                                           After Silicone 10.13 1.746 5.80                                             ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Permeance of wet-spun (0 air gap) and dry-jet wet-spun hollow                   fibers (spun from 35 wt % PES/NMP dopes using 60/40 NMP/H.sub.2 O            as the bore fluid)                                                             Air gap distance                                                                          O.sub.2 permeance                                                                        N.sub.2 permeance                                                                      Selectivity O.sub.2 /N.sub.2                ______________________________________                                        A: Before Silicone Coating                                                      6.0         68         71       0.96                                          0 28 30 0.93                                                                B: After Silicone Coating                                                       6.0         36         37       0.97                                          0 15.9 6.33 2.51                                                            ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Permeance of Wet-spun (0 air gap) and dry-jet wet-spun hollow                   fibers (spun from 30 wt % PES/NMP dopes using 40/60 NMP/H.sub.2 O            as the bore fluid)                                                             Air gap distance                                                                          O.sub.2 permeance                                                                        N.sub.2 permeance                                                                      Selectivity O.sub.2 /N.sub.2                ______________________________________                                        A: Before Silicone Coating                                                      14.4        22.87      25.23    0.91                                          0 75.79 74.9 0.99                                                           B: After Silicone Coating                                                       6.0         12.8       13.5     0.95                                          0 15.0 6.1 2.38                                                             ______________________________________                                    

What is claimed is:
 1. A process for forming ultrathin dense-layerasymmetric hollow fiber membranes comprisinga) dissolving a polymer in asolvent to form a homogenous dope possessing sufficient viscosity andexhibiting chain entanglement at the spinning temperature; b) extrudingthe mixture to form an asymmetric hollow fiber membrane using water asan external coagulant and a bore fluid comprising water and a watermiscible solvent; whereinthe solubility parameter difference between thebore fluid and the spinning dope is less than 2.5 (cal/cm³)⁰.5, and thevolume ratio of bore-fluid flow rate to the dope flow rate ranges from0.45 to 0.75.
 2. The process according to claim 1 wherein the polymer isselected from the group consisting of polyimide, a fluoropolymer,polysulfone, polyethersulfone, polyarylate, polycarbonate, apolybenzimidazole, polyetherketone, polyetherether ketone, andpolyester.
 3. The process of claim 2, wherein the polymer ispolyethersulfone or a fluoropolyimide.
 4. The process of claim 2,wherein the polymer is polyethersulfone.
 5. The process of claim 1,wherein the solvent in a N-(C₁ -C₃ alkyl)-2-pyrolidone.
 6. The processof claim 1, wherein the solvent is N-methyl-2-pyrolidone.
 7. The processof claim 1, wherein the dope is 35/65polyethersulfone/N-methyl-2-pyrolidone.
 8. The process of claim 1,wherein the bore-fluid is 80/20 to 95/5 N-methyl-2-pyrolidone/water. 9.The process of claim 6, wherein the bore-fluid is 80/20N-methyl-2-pyrolidone/water.
 10. The process of claim 7, wherein thebore-fluid is 80/20 N-methyl-2-pyrolidone/water.
 11. The process ofclaim 1 further comprisingc) coating the hollow fiber membrane withsilicone.
 12. The process of claim 9, further, comprisingc) coating thehollow fiber membrane with silicone.
 13. The process of claim 10,further comprisingc) coating the hollow fiber membrane with silicone.